Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia with more than 2 million Americans affected, and is growing exponentially. The major goal of this project is to identify new molecular determinants and novel molecular mechanisms of AF by molecular characterization of the newly-discovered AF gene NUP155. The NUP155 gene encodes a 155 kDa nucleoporin, which is required for the formation of the nuclear pore complex (NPC) and the assembly of the nuclear envelope during mitosis. The NPC is a large macromolecular complex of about 30 nucleoporins, and plays a key role in bi-directional transport of macromolecules with a molecular weight of >40 kDa across the nucleus membrane. Export of mRNA from the nucleus to the cytoplasm plays an important role in gene expression in eukaryotic cells. The NUP155 protein contains a binding domain that interacts directly with mRNA export factor Gle1, which may anchor GLe1 onto the NPC. NUP155 also interacts directly with a NUP53 which forms further complex with other structural nucleoporins. Thus, NUP155 may play an important role in the assembly of the NPC and regulated control of nuclear export of mRNAs. Mutations in NUP155 cause AF. Two NUP155 mutations have been identified, including mutation R391H reported previously by our group (Zhang et al 2008 Cell) and a newly identified mutation H1104P located within the Gle1 binding domain. Homozygous NUP155-/- knockout (KO) mice die before E8.5, but heterozygous NUP155 mice faithfully recapitulate the human AF phenotype. We have demonstrated that atrial myocytes from NUP155 KO mice show significant shortening of action potential duration (APD). However, the molecular mechanisms by which NUP155 mutations cause APD shortening and consequently AF remain unknown. Based on our new preliminary data that IK1 current densities are increased in NUP155 KO atrial myocytes compared to wild type control myocytes, here we propose that the NPC incorporating a mutant NUP155 subunit or less NUP155, or lacking NUP155 becomes defective structurally and/or functionally. The defective NPC may mis-regulate nuclear export of mRNAs for important atrial ion channel genes and/or their regulatory genes (e.g. genes for IK1 subunits Kir2.1, Kir2.2, Kir2.3 or Kir2.x trafficking factors), which leads to abnormal electrical remodeling of ionic currents in the atria (e.g. IK1). Enhanced IK1 and/or other electrical remodeling cause the shortening of APD and shortening of atrial effective refractory period (ERP), and triggers reentry arrhythmias and AF. To test this hypothesis, we will combine cellular and biochemical approaches, electrophysiological studies, computer modeling, and in vivo KO mouse studies to identify new molecular mechanisms of AF. We will first characterize the AF mutations in NUP155 (R391H, H1104P, NUP155 siRNA mimicking KO allele) for their structural effects on the NPC (interaction with Gle1 and NUP53, and complex formation with other nucleoporins, and nuclear envelope localization) as well as for their functional effects on the NPC (nuclear membrane permeability, nuclear export of mRNAs, nuclear import of proteins using Hsp70 as a marker). Secondly, we will use in vivo intracardiac electrophysiological studies to characterize NUP155 KO mice to assess whether the APD shortening at the cellular level is associated with a shortened atrial ERP and increased inducibility of AF at the organ level. The effects of an IK1 specific blocker, gambogic acid, will be evaluated as potential therapy for AF. Finally, we will evaluate the roles of NUP155 in the nuclear export of mRNAs for IK1 subunits, regulation of cell surface trafficking of IK1 subunits, remodeling of IK1 currents, and effects of IK1 blockers on IK1 currents and atrial APD in NUP155 KO mice. In combination with computer modeling, these studies will investigate the functional impact of down-regulation of NUP155 expression on atrial arrhythmias and identify the substrates and important mechanisms for AF cause by the NUP155 mutations. Results obtained from this study will serve our long-term goal of understanding the cardiac-specific signaling by NUP155 in cardiac physiology and disease.